Field of the Invention
This invention relates generally to rubber-metal bonded articles and methods, more particularly to such articles with a self-bonding rubber composition comprising an adhesion promoter selected from a neoalkoxy phosphato zirconate and a polyimide, and specifically to a composition that can be fully cured and formed at one temperature, then the adhesive activated at a higher temperature with minimal further effect on the state of cure.
Description of the Prior Art
Bonding of elastomers to metals is a common practice with many industrial applications. The most common way of metal-elastomer bonding is to use various adhesives, according to various techniques known in the industry. This practice usually involves many preparatory steps for the surfaces involved to ensure adequate bonding. The use of adhesives can involve vulcanization bonding or post-vulcanization bonding.
In rubber-metal composite parts intended for dynamic applications, an important goal is often to maintain the elastomer member in a state of compression between two metal parts in order to ensure good fatigue life. In this case, post-vulcanization bonding is preferred in order to avoid the effect of thermal shrinkage after vulcanization of the elastomer.
Various attempts to reduce the use of adhesives have been made. So-called self-bonding elastomers which bond directly to metal upon vulcanization in the absence of any separate adhesive are known. Generally these elastomers bond specifically to particular metals, such as the elastomer compositions with various cobalt compounds which bond specifically to brass. When self-bonding elastomers are applied in a vulcanization-bonding process, thermal shrinkage may put the elastomer into a state of tension between metal parts.
Sometimes it is advantageous to form the elastomer member in one operation and bond it to the metal members in a separate step, maybe in a separate part of the factory. U.S. Pat. No. 2,409,759 to Hosking, which is hereby incorporated herein by reference in its entirety, teaches a method wherein a partially or fully cured elastomer part is bonded to cuprous metal by confining the elastomer and the metal parts in a second mold and applying heat and hydrostatic pressure sufficient to bond the elastomer to the metal. The cuprous alloy must be polished and the elastomer must contain chemically pure zinc oxide. This method may only partially resolve the shrinkage issue. At best the elastomer element will be in a neutral state upon removal from the confining mold.
A method involving two cure steps is taught by U.S. Pat. No. 7,291,241 to Dunlap, which is hereby incorporated herein by reference in its entirety. The elastomer member is formed first in a shape-forming mold. The partially cured elastomer member is then assembled in a rubber-metal part under compression between two metal members and the vulcanization is completed to effect a bond between rubber and metal. Various coagents for peroxide cured rubber are suggested as effective adhesion promoters. This method partially resolves the shrinkage issue. The rubber element remains in a state of compression after the second cure step.
Since the stress state of an elastomer is an extremely important factor in its durability and also a major contributing factor in its bonding to metal parts, there is a need for high-compression, self-bonding elastomers. Such an elastomer should have a high retained compression and should not need any separate adhesives for bonding to metals.
Co-pending U.S. patent application Ser. No. 12/340,864 filed Dec. 22, 2008, which is hereby incorporated herein by reference in its entirety, is directed to another method of improving the state of compression in a metal-rubber composite article.